Resonant reed relay circuit with long activation time delay



w. c. CLAY 3,153,176

RESONANT REED my cmcun wrm LONG ACTIVATION nus DELAY Oct. 13, 1964 Filed July 3, 1962 United States Patent Oflice 3,153,176 Patented Oct. 13, 1964 3,153,176 RESONANT REED RELAY CIRCUIT WITH LONG ACTIVATION TIME DELAY Wallace C. Clay, Elmhurst, Ill., assignor to the United States of America as represented by the Secretary the Air Force I Filed July 3, 1962, Ser. No. 207,446 2 Claims. (Cl. 317-157) Resonant reed relays are highly selective frequency sensitive devices used in remote control and selective calling systems where the frequency of an audio tone is used to actuate the desired function. When an audio tone of the proper frequency is applied to this type relay, a momentary closure of the output contacts occurs during a portion of each cycle. The contacts are normally connected to a circuit in such a manner that the momentary closures will charge a capacitor. The charge on the capacitor can then be used to turn on a switch such as a transistor or in some cases pull in a secondary D.-C. relay.

In the conventional circuit, there are several undesirable features. One of these is the difiiculty in obtaining long activation time delays that help to prevent functioning on noise signals. Also, the short duration of each contact closure makes it necessary that the capacitor circuit discharge time be longer than the charge time. This is opposite to the desired condition since the probability of intermittent noise pulses charging the capacitor to the activation level is less if the charge time is longer than the discharge time.

The purpose of the invention therefore is to provide a resonant reed relay circuit having a long activation time delay. Briefly, this is accomplished through controlling the output relay of the circuit by means of a long time constant R-C circuit the capacitor of which is normally held in a state of low charge by a shunt resistance. The resonant reed relay is then used to actuate a switch, which may be a diode, to remove the shunt resistor and allow the capacitor to charge slowly to the point where the output relay is actuated. Cessation of the audio tone restores the shunt and rapidly discharges the capacitor of the long time constant circuit.

Referring to the single figure of the drawing for a more detailed description of the invention, the circuit shown responds to audio tones of frequencies f, and f applied simultaneously to input terminals 1 and 2 to actuate relay K the contacts of which may be included in a circuit to be controlled. This relay is actuated by a twostage amplifier, employing transistors Q and Q which has the voltage across C as its input voltage.

In the absence of input signals 7, and f, the contacts of K and K are open and the voltage across C has a low value due to the shunting effect of relatively low resistances R and R which are connected in parallel across C by diodes CR, and CR and the relatively high value of R With the low value of voltage across C the emitter current of Q is below the operating threshold of K the vibrating members 1 and 2 of relays K and K are Input signals having the frequencies f, and f to which resonant cause oscillations of these members of sufficient amplitude to produce periodic closings of the relay contacts. This causes condensers C and C to rapidly charge through the vibrating contacts and current limiting resistor R The resulting increase in potential across C and C back biases diodes CR and CR thus rendering them nonconducting and removing R and R from their shunt connection across C This condenser is now free to charge to the potential of C and C through R at a rate determined by the time constant of the R -C circuit. As the voltage across C increases the Q emitter current through the coil of K increases, eventually actuating this relay. Thus the time interval between the start of the input signals and the actuation of relay K can be made as long as desired by proper choice of the R -C time constant.

When the input signals cease, the charges in C and C are no longer maintained through the K and K contacts so that these condensers rapidly discharge through R and R The fall in the C and C potentials causes diodes CR and CR to again become conductive and C rapidly discharges through R R and these diodes thus releasing relay K While the illustrated embodiment employs two resonant reed relays, the invention is independent of the number of such relays employed. A single relay may be used, but two or more relays increases the security against false operation. Also, any type of amplifier may be usedbetween C and K the transistor circuit shown being merely one example of a suitable amplifier.

I claim:

1. A resonant reed relay circuit having a relatively long time delay, said circuit comprising: a resonant reed relay; an output relay; a source of direct current; a relatively long time constant circuit comprising a first resistor and a first condenser connected in series across said source; a relatively short time constant circuit comprising a current limiting resistor, the contacts of said resonant reed relay and a second condenser connected in series across said source; a third resistance having a value small relative to the value of said first resistance connected in shunt to said second condenser; a switching device arranged to connect said third resistance in shunt to said first condenser whenever the voltage across said second condenser is less than the voltage across said first condenser; and means having an input coupled across said first condenser and an output coupled to said output relay and operative when the voltage across said first condenser exceeds a predetermined value to actuate said output relay.

2. Apparatus as claimed in claim 1 in which said switching device is a diode.

References Cited in the file of this patent UNITED STATES PATENTS 2,567,928 Farmer Sept. 18, 1951 2,807,009 Rowell Sept. 17, 1957 2,994,063 Gibson July 25, 1961 3,027,492 De Vita Mar. 27, 1962 3,099,758 Pieczynski July 30, 1963 

1. A RESONANT REED RELAY CIRCUIT HAVING A RELATIVELY LONG TIME DELAY, SAID CIRCUIT COMPRISING: A RESONANT REED RELAY; AN OUTPUT RELAY; A SOURCE OF DIRECT CURRENT; A RELATIVELY LONG TIME CONSTANT CIRCUIT COMPRISING A FIRST RESISTOR AND A FIRST CONDENSER CONNECTED IN SERIES ACROSS SAID SOURCE; A RELATIVELY SHORT TIME CONSTANT CIRCUIT COMPRISING A CURRENT LIMITING RESISTOR, THE CONTACTS OF SAID RESONANT REED RELAY AND A SECOND CONDENSER CONNECTED IN SERIES ACROSS SAID SOURCE; A THIRD RESISTANCE HAVING A VALUE SMALL RELATIVE TO THE VALUE OF SAID FIRST RESISTANCE CONNECTED IN SHUNT TO SAID SECOND CONDENSER; A SWITCHING DEVICE ARRANGED TO CONNECT SAID THIRD RESISTANCE IN SHUNT TO SAID FIRST CONDENSER WHENEVER THE VOLTAGE ACROSS SAID SECOND CONDENSER IS LESS THAN THE VOLTAGE ACROSS SAID FIRST CONDENSER; AND MEANS HAVING AN INPUT COUPLED ACROSS SAID FIRST CONDENSER AND AN OUTPUT COUPLED TO SAID OUTPUT RELAY AND OPERATIVE WHEN THE VOLTAGE ACROSS SAID FIRST CONDENSER EXCEEDS A PREDETERMINED VALUE TO ACTUATE SAID OUTPUT RELAY. 